Method and apparatus for detecting and treating vulnerable plaques

ABSTRACT

The present technique utilizes microwave radiometry to detect the presence of vulnerable plaques engrained in the wall of a blood vessel. In accordance with the technique, an intravascular catheter containing at least one microwave antenna is moved along the suspect vessel. The antenna, in combination with an external microwave detection and display unit, is able to detect and display thermal anomalies due to the difference in the thermal emissivity (brightness) of vulnerable plaques as compared to normal tissue even though the two may have a common temperature.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a division of Ser. No. 10/452,154, filed Jun. 2,2003, now U.S. Pat. No. 6,932,776.

BACKGROUND OF THE INVENTION

This invention relates to a minimally invasive technique for detectingvulnerable plaques. It relates especially to method and apparatus fordetecting vulnerable plaques utilizing microwave radiometry.

1. Field of the Invention

It is widely known that many heart attacks originate from blockagescreated by athrosclerosis which is the aggressive accumulation ofplaques in the coronary arteries. The accumulation of lipids in theartery and resulting tissue reaction cause a narrowing of the affectedartery which can result in angina, coronary occlusion and even cardiacdeath.

Relatively recent studies have shown that coronary disease can also becaused by so-called vulnerable plaques which, unlike occlusive plaque,are engrained or imbedded in the arterial wall and do not grow into theblood vessel. Rather, they tend to erode creating a raw tissue surfacethat forms caps or scabs. Thus, they are more dangerous than occludingplaque which usually give a warning to a patient in the form of pain orshortness of breath. See, e.g., The Coming of Age of Vulnerable Plaque,Diller, W., Windover's Review of Emerging Medical Ventures, November2000.

2 Description of the Prior Art

Since vulnerable plaques are contained within the vessel wall, they donot result in a closing or narrowing of that vessel. As a result, suchplaques are not easily detectable using conventional x-ray, ultrasoundand MRI imaging techniques.

Moreover, since vulnerable plaques are part of the vessel wall, they mayhave essentially the same temperature as the surrounding normal tissueand the blood flowing through the vessel. Therefore, they are notamenable to detection by known intravascular catheters which rely oninfrared (IR) imaging, thermisters, thermocouples and the like in orderto detect temperature differences in the vessel wall.

Such intravascular catheters are disadvantaged also because they usuallyincorporate an inflatable balloon to isolate the working each end of thecatheter from fluids in the vessel; see for example U.S. Pat. No.6,475,159. As seen there, the IR detector is located within the balloonwhich constitutes an insulating (not transparent at IR frequencies)layer between the detector and the vessel wall causing significantattenuation of the signal from the detector. Also, the undesirablestoppage of blood flow by the balloon increases the risk to the patient.Still further, the balloon has to be repeatedly inflated and deflated inorder to image different locations along the blood vessel increasing theoperating time during which the patient is at risk.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide amethod of detecting vulnerable plaques before the plaques rupture andcause thrombosis.

Another object of the invention is to provide such a vulnerable plaquedetection method which does not require the stoppage of blood flow inthe affected vessel.

An additional object of the invention is to provide a method ofdetecting vulnerable plaques using microwave radiometry.

Another object of the invention is to provide intracorporeal microwaveapparatus for detecting vulnerable plaques having one or more of theabove advantages.

A further object of the invention is to provide such apparatus capableof treating the plaques after detection.

Other objects will, in part, be obvious and will, in part, appearhereinafter.

The invention accordingly comprises the several steps and the relationof one or more of such steps with respect to each of the others, and theapparatus embodying the features of construction, combination ofelements and arrangement of parts which are adapted to effect suchsteps, all as exemplified in the following detailed description, and thescope of the invention will be indicated in the claims.

Briefly, the present method utilizes microwave radiometry to detect thepresence of vulnerable plaques engrained in the wall of a blood vessel.In accordance with the method, an intravascular catheter containing atleast one microwave antenna is moved along the suspect vessel. Theantenna, in combination with an external microwave detection and displayunit, is able to detect and display thermal anomalies due to thedifference in the thermal emissivity (brightness) of vulnerable plaquesas compared to normal tissue even though the two may have a commontemperature. In other words, it has been found that the microwavecharacteristics of vulnerable plaques imbedded in a vessel wall aredifferent from those of normal tissue comprising the vessel wall andthis difference is detected as a thermal anomaly and displayed orplotted as the catheter is moved along the vessel.

As we shall see, in some applications the detected plaques may then bytreated by microwave ablation using the very same catheter.

In its simplest form, the microwave antenna may be a more or lessconventional microwave antenna located at the distal or working end ofthe catheter. The inner and outer conductors of the antenna areconnected by a coaxial cable to an external detection and display unitwhich detects the microwave emissions from the blood vessel picked up bythe antenna and produces corresponding output signals for a displaywhich responds to those signals by displaying the thermal emissions fromthe blood vessel in real time as the catheter is moved along the vessel.

In accordance with the invention, the radiometer is preferably a Dickeswitch-type radiometer and the temperature of the blood flowing throughthe vessel, which corresponds to the body's core temperature, is used asthe radiometer reference. The operating frequency of the radiometer isselected to detect microwave emissions from a depth in the blood vesselwall where vulnerable plaques are likely to be imbedded, e.g. afrequency in the range of 1 to 4 GHz, preferably 1 GHz. Thus as thecatheter is moved along the vessel, it is maintained at a constantbackground or core temperature corresponding to the temperature of theblood and of normal tissue. The locations of vulnerable plaques aredetected as thermal anomalies (hot spots) due to the higher emissivityof the plaques as compared to normal tissue. Using the output of theradiometer to control a display, the plaque sites along the vessel canbe plotted.

It is important to note that the present method and apparatus allow thedetection of vulnerable plaques at subsurface locations in the vesselwall without contacting the vessel wall and without any interruption ofblood flow through the vessel.

As will be described in more detail later, the catheter may include alengthwise passage for receiving a guide wire to help guide the catheterinto and along the blood vessel being examined. As we shall see, in someapplications the guide wire itself may actually constitute the innerconductor of the antenna within the catheter. Also, in order to helpcenter the antenna within the blood vessel, the catheter may incorporatean expandable perforated standoff device which spaces the antenna fromthe vessel wall without materially interfering with the blood flowthrough the vessel.

In a preferred embodiment of the invention, the detection unit includestwo radiometers operating at different frequencies. One radiometer,operating at a higher frequency in the range of 3 to 6 GHz, preferably 4GHz, detects thermal emissions from the inner surface of the bloodvessel. This temperature, corresponding to the body core temperature, isused as a reference. The second radiometer operates at a lower frequencyof 1 to 4 GHz, preferably 1 GHz, to detect thermal emissions fromsubsurface locations in the vessel wall which may contain embeddedvulnerable plaques. Thus by subtracting the outputs of the tworadiometers, the sites of vulnerable plaques can be detected anddisplayed continuously and in real time as the catheter is moved alongthe blood vessel.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the nature and objects of the invention,reference should be made to the following detailed description taken inconnection with the accompanying drawings, in which:

FIG. 1 is a diagrammatic view of apparatus for detecting vulnerableplaques in accordance with the invention and employing a firstintravascular catheter embodiment;

FIG. 2 is a fragmentary sectional view of a second catheter embodimentfor use with the FIG. 1 apparatus;

FIG. 3 is a similar view showing a third catheter embodiment;

FIG. 4 is a view similar to FIG. 1 of another apparatus embodiment, and

FIG. 5 is a sectional view of the diplexer component of the apparatus.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Referring to FIG. 1 of the drawings, the present apparatus comprises acatheter shown generally at 10 for insertion into a blood vessel V whichmay have locations where vulnerable plaques P are embedded or engrainedin the vessel wall. Such plaques P typically include a relatively largeportion of the vessel wall, e.g. a third to a half of its circumference.Catheter 10 is connected by coaxial cables 12 a and 12 b to a detectionand display unit 14. The catheter has a proximal end 10 a to whichcables 12 a and 12 b are connected by way of a fitting or connector 16and a distal end or tip 10 b. The catheter may have a length of 100 cmor more and is quite narrow and flexible so that it can be threadedalong a conventional introducer, e.g. 8.5 French, allowing the distalend 10 b of the catheter to be placed at a selected position in apatient's blood vessel V. Typically, vessel V is accessed by a vein inthe patient's neck or groin.

The catheter may include an expandable perforated stand-off device suchas shown U.S. Pat. No. 6,496,738, which is hereby incorporated herein byreference, so as to center the catheter 10 in vessel V without impedingthe blood flow through that vessel.

As shown in FIG. 1, catheter 10 comprises a central conductor 22surrounded by a cylindrical layer 24 of a suitable low loss dielectricmaterial. Surrounding the layer 24 is a tubular middle conductor 26surrounded by a dielectric layer 28. Finally, a tubular outer conductor30 encircles layer 28. At fitting 16, the proximal ends of conductors22, 26 and 30 are connected by way of a passive diplexer 31 (FIG. 5) tothe coaxial cables 12 a and 12 b. Preferably, the catheter has aprotective outer coating, e.g. of PTFE, (not shown).

At the distal end segment of catheter 10, the middle conductor 26extends beyond the outer conductor 30 to form a microwave antenna 32which, in this case, is a monopole as in the above U.S. Pat. No.6,496,738. In some applications, a helical antenna or capacitive tip maybe used; see U.S. Pat. Nos. 4,583,556 and 4,557,272, the contents ofwhich are hereby incorporated herein by reference.

The distal end 10 b of the catheter is actually formed by a rounded lowloss dielectric button 34 which is butted and secured to the distal endof the dielectric layers 24 and 28. Typically, the middle conductor 26extends beyond the outer conductor 30 a distance in the order of 1 cm sothat antenna 32 has a relatively long antenna pattern. Also, if desired,the diameters of the coaxial conductors in catheter 10 may be steppeddown along the catheter to improve antenna performance. Antenna 32detects the thermal radiation emitted from blood vessel V and applies acorresponding electrical signal via cable 12 a to a radiometer 38 inunit 14. The radiometer 38 may be a conventional Dicke switch-typeradiometer as described in the above U.S. Pat. No. 4,557,272. Thetemperature-indicating signal from antenna 32 is applied via cable 12 ato the signal input 40 a of the Dicke switch 40 in radiometer 38. Theother input to the switch 40 is a reference value which corresponds tothe patient's core temperature, e.g. 37° C.

The temperature may be measured using a resistive termination or load orheat sensor 42 connected between inner conductor 22 and middle conductor26 near the catheter tip. The sensor output or value is applied viathose conductors to diplexer 31 in connector 16 which separates thatsignal from the antenna signal. That reference signal is thereuponconducted by cable 12 b to the reference input 40 b of switch 40. Inother words, two ports of the radiometer are brought out to receive boththe antenna and reference signals from catheter 10. The advantage ofthis arrangement is that the unknown temperature is now compared withthe actual blood (core) temperature. This improves radiometersensitivity (performance) by keeping all circuitry that precedes theDicke switch at the same temperature.

The radiometer operates at a center frequency in the order of 1 to 4 GHzso that the apparatus can detect thermal emissions from locationsrelatively deep in the wall of vessel V.

The output of the radiometer 38 is processed by a processor 44 in unit14 which controls a display 46.

To use the FIG. 1 apparatus, the catheter 10 is threaded into thepatient's vessel V in the usual way. After insertion, the catheterassumes essentially the same temperature as the vessel V and the bloodflowing through the vessel. This temperature as sensed by sensor 42 isused as the reference for Dicke switch 40 which toggles between itssignal and reference inputs 40 a and 40 b in the usual way. When thecatheter is moved along the vessel V, say, in the direction of the arrowA, the antenna 32 will pick up thermal emissions from the normal tissuein the vessel wall and unit 14 will provide a core or back-groundtemperature indication which will be displayed by display 46. However,when the antenna 32 is moved opposite a region containing vulnerableplaques P, the apparatus will detect a thermal anomaly due to theincreased emittance (brightness) of the plaques embedded in the vesselwall. Thus, as the catheter 10 is moved along the vessel, the unit 14can display continuously in real time the locations of plaques P as wellas other useful information such as the body's core temperature,diagnostic data and the like as instructed via the processor's keyboard44 a.

Referring now to FIG. 2, in some procedures, it may be desirable thatthe catheter be guided along the blood vessel V by means of a guidewire. FIG. 2 illustrates an intravascular catheter shown generally at 50capable of being moved along a guide wire W previously introduced intothe blood vessel in a conventional manner. Catheter 50 is similar tocatheter 10 in FIG. 1 except that its central conductor 54 is anelongated flexible conductive tube. The other parts of catheter 50 aremore or less the same as those of catheter 10 and therefore carry thesame identifying numerals.

In catheter 50, conductor 54 extends through the fitting 16 as well asall the way through the button 34 to the tip 10 b of the catheter. Thisallows the guide wire W to be threaded through the tubular conductor 54so that the catheter 50 can be moved along the guide wire after theguide wire has been introduced into the blood vessel being examined.

When the catheter 50 is in use, the guide wire W does not interfere withthe antenna pattern of antenna 32 because it is shielded by conductor54. In other words, the field around the antenna does not extend withinthe metal conductor 54.

In some applications, the guide wire W itself may be used as the centralconductor of the antenna 32 in the catheter. FIG. 3 shows such acatheter at 60 which may be used to detect vulnerable plaques asdescribed above. As shown there, the catheter 60 is similar to catheter50 except that it is devoid of the tubular central conductor 54.Instead, it is formed with an axial passage 62 in dielectric layer 24and button 34 which passage extends snugly but slidably all the way fromthe tip of the catheter to the proximal end thereof and through thefitting 16 so that the guide wire W can be threaded through passage 62as shown. In this case, the guide wire itself is connected electricallyvia cable 12 b to the detection and display unit 14. In use, the guidewire may be introduced into the blood vessel to be examined and thenremain stationary while the remainder of the catheter is slid along theguide wire in order to examine different lengthwise segments of theblood vessel wall. Alternatively, the entire catheter 60 including theguide wire W may be moved as a unit along the blood vessel in order toadvance the antenna 32 along that vessel.

Refer now to FIG. 4 which shows a preferred embodiment of the inventionthat can detect even very small thermal anomalies in the vessel wall dueto embedded or engrained plaques. The FIG. 4 apparatus comprises acatheter shown generally at 70 having coaxial inner and outer antennasindicated at 72 a and 72 b. The inner antenna 72 a comprises an innerconductor 74 and an outer conductor 76 separated by an insulating layer78. The inner conductor 74 extends beyond the outer conductor 76 formingthe antenna 72 a.

The outer antenna 72 b comprises a tubular inner conductor 82 and anouter conductor 84 separated by an insulating layer 86, the innerconductor 82 extending beyond the outer conductor 84 to form the antenna72 b. The proximate end of catheter 70 is terminated by a fitting orconnector 88 containing a diplexer 31 (FIG. 5) which connects theconductors of the antennas 72 a, 72 b to coaxial cables 90 a and 90 bleading to a detection and display unit 92.

In some applications, the outer conductor 76 of antenna 72 a and theinner conductor 82 of antenna 72 b may be a common conductor. Morepreferably those conductors are separate as shown so that the innerantenna 72 a is slidable within the outer antenna 72 b and fitting 88 sothat the distance D between the two antennas can be varied from zero toseveral centimeters allowing the outer and inner antennas to beoptimized at two specific frequencies F₁ and F₂. The inner conductor 74of antenna 72 a may be hollow or tubular so that it can receive a guidewire as described in connection with FIG. 2. Alternatively, thatconductor may be sufficiently small to serve as the guide wire itself asdescribed in connection with FIG. 3.

In order to electrically separate the outputs of the two antennas 72 aand 72 b, the fitting or connector 88, like connector 16, incorporates apassive diplexer 31. As seen from FIG. 5, the diplexer includes aquarter-wave (λ/4) stub 91 to bring out the signal F₂ from the innerantenna 72 a. This stub also provides a matched 90° bend to separate andbring out the signal F₁ from the outer antenna 72 b.

Whereas it is known in the art to use a quarter-wave stub to support thecenter conductor of an antenna, the present diplexer has a tubular innerconductor 94 which receives the coaxial cable 74-78 comprising the innerantenna 72 a providing signal F₂. That conductor 94 may be an extensionof the antenna conductor 82. Surrounding and insulated from conductor 94is a coaxial outer conductor 96 which may be an extension of antennaconductor 84. The two diplexer conductors 94 and 96 are shorted by anend plate 98 at the end of stub 91. Conductor 94 has a branch 94 a whichis brought out through a branch 96 a of conductor 96 to deliver thesignal F₁ from antenna 72 b. Preferably, the coaxial cable 74-78 isslidable to some extent along conductor 94 to vary the antenna distanceD as described above.

The illustrated diplexer 31 provides several distinct advantages. Itseparates the concentric cables from antennas 72 a and 72 b in FIG. 4into two separate cables; allows those cables to be mechanically andindependently positioned, and it allows the innermost conductor todouble as a guide wire for the catheter as shown in FIGS. 2 and 3.

In the FIG. 4 apparatus, the smaller diameter antenna 72 a, optimized ata frequency F₂, e.g. 3-6 GHz, may measure the blood and normal tissuetemperature, whereas the larger diameter antenna 72 b optimized atfrequency F₁, e.g. 1-4 GHz, measures the temperature of the deepertissue where vulnerable plaques are likely to occur. This largerdiameter provides a less lossy cable making that antenna more efficient.The larger diameter antenna also places it closer to the wall of vesselV (FIG. 1), further increasing the depth of detection.

While the catheter 70 in FIG. 4 could be connected by cables 90 a and 90b to the switch 40 of a single radiometer as shown in FIG. 1, theillustrated detection and display unit 92 contains two radiometers 38 aand 38 b connects to cables 90 a and 90 b, respectively. The formerwhich operates at the frequency F₁ detects thermal anomalies picked upby antenna 72 b due to plaques located relatively deep in the wall ofvessel V (FIG. 1) as before; the latter operating at frequency F₂detects thermal emissions picked up by antenna 72 a at the inner surfaceof the vessel which reflect the body core temperature. The processor 44thereupon subtracts the signals and causes display 46 to display thelocations of thermal anomalies which are likely due to plaques P.

After the vessel V has been examined and the locations of the plaques Pdetermined as described above, the plaques P may be treated by microwaveablation using the very same catheter. This may be done by connectingthe catheter via a diplexer to a microwave transmitter in order to heatthe plaques as described in the above U.S. Pat. No. 6,496,738.

It will thus be seen that the objects set forth above, among those madeapparent from the preceding description, are efficiently attained and,since certain changes may be made in carrying out the above method andin the constructions set forth without departing from the scope of theinvention, it is intended that all matter contained in the abovedescription or shown in the accompanying drawings shall be interpretedas illustrative and not in the limiting sense.

Is also to be understood that the following claims are intended to coverall of the generic and specific features of the invention describedherein.

1. A method of detecting vulnerable plaques engrained or embedded inotherwise normal tissue in a wall of a patient's blood vessel containingblood whose temperature corresponds to that of said normal tissue, saidmethod comprising the steps of forming an intravascular cathetercontaining a first microwave antenna tuned to a first frequency enablingit to detect temperature at depth in said wall and produce acorresponding first frequency signal, and a second antenna designed topick up thermal emissions from said blood to sense the temperaturethereof and produce a corresponding reference signal; applying saidsignals by way of a diplexer separately to a differential radiometer toproduce an output signal therefrom which reflects said temperature atdepth relative to the blood temperature, and moving the catheter alongsaid vessel so that at each point along the vessel said output signalindicates the presence or absence of vulnerable plaques in said vesselwall due to the higher emissivity thereof as compared to that of thenormal tissue.
 2. The method defined in claim 1 and including the stepof using said output signal to control a display device to display thelocations of said vulnerable plaques in said blood vessel wall. 3.Apparatus for detecting vulnerable plaques engrained or imbedded inotherwise normal tissue in a wall of a patient's blood vessel containingblood whose temperature is substantially the same as that of said normaltissue, said apparatus comprising an intravascular catheter containing amicrowave antenna tuned to a first frequency enabling it to detecttemperature at depth in said wall and produce a corresponding firstfrequency signal; a radiometer having a signal input, a reference inputand an output, a diplexer connecting the antenna to the signal input; asecond antenna in the catheter for sensing the temperature of the bloodand producing a corresponding reference signal, and means for applyingthe reference signal to the reference input, said catheter being movablealong said vessel so that at each axial location along the vessel, saidoutput signal indicates the presence or absence of vulnerable plaquesdue to the higher emissivity thereof as compared to that of the normaltissue.
 4. The method defined in claim 1 wherein said second antenna istuned to a second frequency that is different from said first frequency.5. The method defined in claim 4 wherein the first frequency is in therange of 1-4 GHz and the second frequency is in the range of 3-6 GHz. 6.The method defined in claim 1 wherein the first and second antennas aremade coaxial with the second antenna being radially inside, andextending axially beyond, said first antenna.
 7. A method of detectingvulnerable plaque ingrained or imbedded in otherwise normal tissuecomprising a wall of a patient's blood vessel containing blood whosetemperature is substantially the same as that of said normal tissue,said method comprising the steps of forming an intravascular cathetercontaining a first microwave antenna tuned to a first frequency enablingdetection of the temperature at depth in said wall and a secondmicrowave antenna tuned to a second frequency enabling the detection ofthe temperature of said blood; providing a first radiometer having anoperating frequency range which includes said first frequency and withan input connected by way of a diplexer to the first antenna and whichproduces a first output signal representing said temperature at depth;providing a second radiometer having an operating frequency range whichincludes said second frequency and with an input connected by way ofsaid diplexer to the second antenna and which produces a second outputsignal representing the temperature of said blood; producing adifference signal from said first and second output signals, and movingthe catheter along said vessel so that at each axial location along thevessel, said difference signal indicates the presence or absence ofvulnerable plaque in said vessel wall due to the higher emissivity ofsaid plaque as compared to the normal tissue.
 8. The method defined inclaim 7 wherein the first frequency is in the range of 1-4 GHz and thesecond frequency is in the range of 3-6 GHz.
 9. The method defined inclaim 1 including the step of heating said plaques following theirdetection.
 10. The apparatus defined in claim 3 and further including adisplay device responsive to the difference signal for displaying thelocations of the vulnerable plaques in said blood vessel wall.
 11. Theapparatus defined in claim 3 wherein the first and second antennas arecoaxial with the second antenna being radially inside, and extendingaxially beyond, the first antenna.
 12. The apparatus defined in claim 11wherein the first and second antennas are slidable axially relative toone another.
 13. The apparatus defined in claim 11 wherein the secondantenna comprises a tube with an axial passage for accepting a guidewire.
 14. The apparatus defined in claim 3 wherein the second antennaincludes a resistive termination or load.
 15. The apparatus defined inclaim 3 and further including means for heating said plaques followingthe detection thereof.
 16. The method defined in claim 6 including thestep of forming the second antenna with an axial passage for receiving aguide wire.
 17. The method defined in claim 6 including the step ofmaking the first and second antennas slidable axially relative to oneanother.
 18. The method defined in claim 6 including the step ofproviding the second antenna with a resistive termination or load. 19.The method defined in claim 7 including the additional step of applyingsaid difference signal to control a display device to display thelocations of said plaque in the blood vessel wall.